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FS ShieldForest Health Protection, Southern Region

COTTONWOOD BORER,

Plectrodera scalator (Fab.)

Importance. - The cottonwood borer ranges throughout the eastern Unites States, but highest populations and greatest damage occur in the South. It attacks cottonwood and willow. Trees weakened by severe infestations may be broken off by wind. Damage is sometime serious in cottonwood nurseries, natural stands, and plantations, particularly those planted offsite.

Identifying the Insect. - Adult beetles are 1 to 1 1/5 inches (25 to 38 mm) long and about 1/2 inch (12 mm) wide. They are black with lines of cream-colored hair forming irregular black patches. Larvae are seldom seen.


Adult
Adult.


Identifying the Injury. - The adults may cause serious damage in cottonwood nurseries by feeding on the tender shoots of young trees, causing them to shrivel and break off. The larvae bore into the inner bark and wood at the root collar and tunnel downward into the roots. Light brown, fibrous frass is sometimes ejected from bark openings at or slightly above the ground line, accumulating in piles at the base of the tree. The root collar and roots of infested trees may be riddled by larval tunnels.

Biology. - The adults appear in midsummer. After feeding briefly, they descend to the bases of host trees where the female deposits her eggs in small pits gnawed in the bark. Eggs hatch in 16 to 18 days. The larvae bore downward in the inner bark, entering a large root by autumn. Pupation occurs in the gallery from April to June and lasts about 3 weeks. The new adults chew exit holes through the sides of the pupal chambers and emerge through the soil. Some larvae complete development in 1 year, while others require 2 years.

Control. - Management practicessuch as locating new nurseries away from infested trees, planting uninfested cuttings, and removing and destroying infested rootstock-help to minimize damage. Three weekly applications of insecticide, timed to begin soon after emergence, have given good control of adult beetles in nurseries.


Larvae in a root

Larvae in a root. (Click for detail. JPG 27K).